The invention relates to a vehicle heater which, in a vehicle heating circuit, is connected to the coolant circuit of the internal combustion engine, but is operable as an auxiliary heater independent of the engine, and is installed downstream of a vehicle heat exchanger. The vehicle heat exchanger radiates heat to the vehicle interior. The auxiliary heating apparatus, independent of the engine, has a built-in temperature sensor for detection of the temperature of the heat exchange medium of the heating apparatus and for controlling the heating apparatus.
In the case of vehicle heaters of the above-named type which throttle the rate of flow of coolant in the heating circuit, optionally until cutoff, the coolant temperature in the heating circuit outside the internal combustion engine can drop, which can result in the engine independent heating apparatus, provided in a so-called in-line installation, being turned on and put into operation, although there is actually only a small heat requirement or none at all. Since the heating apparatus, independent of the engine, works in such cases and produces heat, this heat cannot be removed by the heating circuit of the vehicle and the heating apparatus can reach critical operating conditions, so that, for example, safety devices, such as a temperature safety device or the like, are tripped.
Therefore, a primary object of the present invention is, to overcome the difficulties mentioned above and, thereby, to improve a vehicle heater of the type initially mentioned so that, the engine independent heating apparatus, is activated or turned on only if there is actually a heat requirement in the heating circuit of the vehicle heater.
In this connection, according to the invention a vehicle heater of the type initially mentioned is distinguished by the fact that a temperature sensor, in addition to the built-in sensor, is provided on the vehicle heat exchanger for detecting the output temperature, and that the magnitude of a temperature difference value determined from the temperature values detected by the temperature sensors can be utilized as a control parameter.
Thus, since, with the vehicle heater according to the invention, the temperature of the liquid in the heating circuit is detected at the output of the vehicle heat exchanger, by comparison of the value from the temperature sensor for the detection of the heat exchange medium temperature in the heating apparatus with the temperature in the output area of the vehicle heat exchanger, it is possible to detect precisely whether or not a decrease in temperature occurred through the vehicle heat exchanger. If this established value of temperature difference does not exceed a predetermined value, this means that a very small or no heat requirement at all exists, so that the auxiliary heating apparatus, is not turned on or the heating-circuit connected.
Thus, if a predetermined temperature difference between the two temperature sensors is reached or is lower, a heating requirement exists and the auxiliary heating apparatus, is put into operation.
With the vehicle heater according to the invention, the auxiliary heating apparatus is kept from going into operation if the temperature sensor provided on the auxiliary heating apparatus for detection of the temperature of the exchange medium goes below the turn-on temperature for the heating apparatus, but still no temperature decrease or only a very small temperature decrease takes place in the heating circuit of the vehicle heater. In this way, critical operating conditions of the auxiliary heating apparatus, especially in the case of an in-line installation in a vehicle heater, are avoided.
In accordance with another aspect of the vehicle heater according to the present invention, the auxiliary heating apparatus remains in a stand-by mode as long as a predetermined threshold value for the temperature detected by the additional, heat exchanger associated, temperature sensor is not reached or exceeded. By this means the condition of the vehicle heater at which, for example, a valve located upstream of the vehicle heat exchanger is cut off, is taken into account so that the liquid on the intake side and the output side of the heat requirement cannot be made with sufficient reliability solely by consideration of the temperature difference between the two temperature sensors. Therefore, in this case, a threshold value is preset for the temperature detected by the second temperature sensor.
In an advantageous further development according to the invention, the output of the additional temperature sensor is connected to a control device for control and regulation of the engine independent heating apparatus, and the evaluation takes place directly with the help of this control device.
Further, the predetermined threshold value for the temperature detected by the additional temperature sensor can be preset in the control device, so that it is only necessary to connect the additional temperature sensor on the output area of the vehicle heat exchanger to the control device of the heating apparatus. As a result, the installation expense, even in the case of a retrofit installation, can be minimized.
Also, according to the invention, the design of the vehicle heater is also made so that the predetermined threshold value for the temperature detected by the additional temperature sensor is considered only if the heating apparatus has cut off once, to guarantee a heating operation with the use of the heating apparatus, even in the case of a cold internal combustion engine.
These and further objects, features and advantages of the present invention will become more obvious from the following description when taken in connection with the accompanying drawings which show, for purposes of illustration only, single embodiments in accordance with the present invention.